In the conventional multichannel audio signal coding field, many studies have been made on coding using correlation between stereo signals. One known method is to group five channels, for example, in pairs to render them to coding of stereo signals. Compressive coding is also often used which exploits similarity between channels on the basis of the difference or a fixedly weighted difference signal between the channels. However, most compressive coding techniques provide low compression efficiencies. An example of the technique is disclosed in Non-patent literature 1.
The conventional predictive coding will be described with reference to FIG. 1. As shown in FIG. 1A, at the coding end, a time-series digital signal provided through an input terminal 11 is divided by a frame divider 12 into short-time periods (called frames) consisting of a predetermined number of samples. A linear prediction analyzing section 13 performs linear prediction analysis on each frame of the digital signal to compute prediction coefficients. The prediction coefficients are typically quantized by a quantizing section 13a in the linear prediction analyzing section 13.
The quantized prediction coefficients and the digital signal of the frame are inputted into a linear predicting section 14. The linear predicting section 14 performs linear prediction of the digital signal in the time direction to obtain a prediction value of each sample. The linear prediction is autoregressive forward prediction. A subtracting section 15 subtracts the prediction value from the corresponding sample of the input digital signal to generate a prediction error signal. The linear predicting section 14 and the subtracting section 15 constitute a prediction error generating section 16.
The prediction error signal from the prediction error generating section 16 is entropy-coded in a compressive coding section 17 using Huffman coding or arithmetic coding and the result is outputted as an error code. The quantized prediction coefficients from the linear predictive analyzing section 13 are coded in a coefficient coding section 18 using entropy coding or vector quantization and the result is outputted as a coefficient code. The quantized prediction coefficients may be outputted intact as scalar-quantized prediction coefficients.
As shown in FIG. 1B, expansion-decoder 21 at the decoding end decodes an inputted compressed code by using a decoding scheme corresponding to a coding scheme used by the compressive coding section 17 to generate a prediction error signal. A coefficient decoder 22 decodes an inputted coefficient code using a decoding scheme corresponding to a coding scheme used by the coefficient coding section 18 to generate prediction coefficients. The decoded prediction error signal and prediction coefficients are inputted into a predictive synthesizing section 23, where they are predictive-synthesized to reproduce a digital signal. A frame synthesizing section 24 sequentially combines the digital signal of each frame and outputs the combined digital signal to an output terminal 25. In the predictive synthesizing section 23, the digital signal to be reproduced and the decoded prediction coefficients are inputted into a regressive linear prediction section 26, where a prediction value is generated, and the prediction value and the decoded prediction error signal are added together in an adder 27 to reproduce the digital signal.
Non patent literature 1: “An Introduction to Super Audio CD and DVD-Audio”, IEEE SIGNAL PROCESSING MAGAZINE, July 2003, pp. 71-82.